To detect an acoustic signal downhole, distributed acoustic sensing (DAS) is commonly and effectively used. This method employs fibre optic cables to provide distributed acoustic sensing whereby the fibre optic cable acts as a string of discrete acoustic sensors, and an optoelectronic device measures and processes the returning signal. The operation of such a device is described next.
A pulse of light is sent into the optical fibre, and a small amount of light is naturally back scattered, along the length of the fibre by Rayleigh, Brillouin and Raman scattering mechanisms. The scattered light is captured by the fibre and carried back towards the source where the returning signal is measured against time, allowing measurements in the amplitude, frequency and phase of the scattered light to be determined. If an acoustic wave is incident upon the cable, the glass structure of the optical fibre is caused to contract and expand within the vibro-acoustic field, consequently varying the optical path lengths between the back scattered and/or reflected light scattered from different locations along the fibre The returning signal can be processed in order to measure the acoustical and/or vibrational field(s) at all points along the structure.
In known distributed acoustic sensing systems (DAS), standard fibre optic cables are utilised to obtain a measurement profile from along the entire length of the fibre at intervals ranging from 1-10 meters. Further details regarding the operation of a suitable DAS system, such as the iDAS™, available from Silixa Limited, of Elstree, UK are given in WO2010/0136809. Systems such as these are able to digitally record acoustic fields at every interval location along an optical fibre at frequencies up to 100 kHz. Since the location of the acoustic sensors is known (the fibre deployment being known), the position of any acoustic signal can be thus identified by means of time-of-arrival calculations.
DAS systems find lots of applications in the oil and gas industry, and optical fibers that can be connected to DAS systems, amongst other things, are often installed within wellbores, usually as a metal cable running parallel with the well bore casing clamped to the outside thereof. In a typical oil or gas well, once the well bore has been drilled and the casing installed, cement is used to fill the well bore external of the casing. However, as part of the “completion” process of the well, the casing and cement is perforated within the hydrocarbon bearing regions, to allow hydrocarbons to flow into the casing for extraction. Perforation is typically performed by a perforating gun, which is typically a cylindrical metal tube provided with shaped explosive charges arranged around the circumference thereof. The perforating gun is lowered through the casing to the intended production zone, and the shaped charges are detonated, with the intention of blasting holes through the casing and cement of the well, and into the surrounding rock strata, to allow hydrocarbons to then flow through the created channels into the casing for extraction. Similarly, where a fracturing fluid is to be pumped into the well to fracture the rock strata, the created holes provide routes for the fracturing fluid to exit the well into the surrounding rock.
FIG. 11 illustrates the use of a perforating gun to generate perforations in a well bore casing and cement, and into the surrounding rock strata. Perforating gun 10 comprises a metal cylinder provided with shaped explosive charges arranged around the outer surface thereof. For example, the shaped charges may be provided in lines every 120 degrees around the outer circumference of the gun. The gun is provided with a communications line 12 to the surface for control purposes, to allow the explosive charges to be detonated on command. In use as noted above the gun is lowered to the intended production zone, and the shaped charges detonated to blast through the casing and cement (as shown in FIG. 11(b)), to create production channels in the surrounding rock strata through which oil or gas can flow to enter the well bore (as shown in FIG. 11 (c)).
One issue with the use of perforating guns is to try and prevent the shaped charges from damaging any control or sensing cabling or other lines that may extend along the wellbore external of the casing. For example, optical fibers are commonly installed along the external surface of the casing within the wellbore, either for sensing purposes and/or for control of downhole tools. Care must be taken when using a perforating gun that the shaped charges are not pointed at the external cabling or other lines such that the charges when detonated would sever such lines. As the perforating is performed as part of the well completion, by that point the fibers have typically already been cemented into the well bore, and hence repair can be very costly, or even impossible. To try and prevent such damage occurring, conventionally the fibers and other signalling lines are located between two metal rods or cables, and a magnetometer is provided on the perforating gun to try and detect the metal rods. That is, the rotational orientation of the perforating gun is altered within the casing whilst the magnetometer is used to detect the location of the metal rods either side of the fibers or other cabling. Once the metal rods have been detected, the orientation of the perforating gun can be controlled to ensure that the shaped charges are pointed away from the area of the metal rods, and hence the cabling or other lines to be protected.
One problem with the above arrangement is one of cost, in that the metal rods are usually required to extend along a significant length of the well bore, hence increasing the material and production cost of the well. In addition, the use of magnetometers to detect the rods is not particularly accurate, and particularly in some rock formations or in some regions where magnetic anomalies can occur that interfere with the operation of the magnetometers. Moreover, the presence of the casing and other downhole equipment can interfere with the proper operation of the magnetometers, meaning that it is not reliably possible to rotationally orient the perforating gun within the casing to ensure that the sensor and control lines and/or other cabling will not be damaged by the use of the perforating gun. In addition, the rods also form a potential leakage path up the outside of the casing.
In order to address this problem WO2013/030555 describes a method and apparatus for determining the relative orientation of objects downhole, and especially to determining perforator orientation. The method, illustrated in FIG. 12, involves varying the orientation of an object, such as a perforator gun (302) in the wellbore and activating at least one directional acoustic source (402a-c). Each directional acoustic source is fixed in a predetermined location to the object and transmits an acoustic signal preferentially in a known direction. The directional acoustic source(s) is/are activated so as to generate sound in a plurality of different orientations of said object. An optical fiber (104) deployed down the wellbore is interrogated to provide distributed acoustic sensing in the vicinity of the object and the acoustic signals detected by the optical fiber are analyzed so as to determine the orientation of the at least one directional acoustic source relative to the optical fiber, for instance by looking at the relative intensity in the different orientations. Further details of the operation of the arrangement are described in the document, any and all of which necessary for understanding the present invention being incorporated herein by reference.
Therefore, whilst the arrangement in WO2013/030555 apparently should overcome the cost and inaccuracy of the prior art magnetometer arrangements, the arrangement relies on the operation of a DAS system to detect the directional acoustic sources, with the directional acoustic sources being described as conventional loudspeakers arranged to project sounds forward and located in a casing that absorbs sound emitted in other directions. Conventional loudspeakers typically operate within audible frequency bands, for example in the range 20 Hz to 20 kHz, and a typical DAS of the prior art is usually capable of detecting sound at these frequencies with good spatial resolution. However, the directionality of conventional loudspeakers, even provided in an otherwise insulating casing, is not high, and −3 dB directivity arcs of +/−50 to 60° can be common. FIG. 12 has been annotated to show typical example-directivity arcs for the three loudspeakers. As shown, such directivity often means that even if the speaker is pointed away from the optical fibre, the fiber may still pick up a large signal from the speaker. Allowing further for echoes and other multi-path effects within the casing, and the reliability of such a system begins to deteriorate. Basically, using conventional speakers as described in the prior art does not give a high enough directivity for the sound emitted to reliably determine the orientation of the perforating gun.